Chapter 10 – Heredity and Common Genetic Diseases

Do you want to make this chapter easy to understand before your exam? The topic Heredity and Common Genetic Diseases is important for your WBBSE Class 10 Geography exam. This page gives you important questions and answers in clear and simple English. These questions are selected from important topics and exam patterns. This will help you understand better, revise faster, and score good marks in your Madhyamik exam.

Content Structure

1. Multiple choice questions and answers (MCQs)
2. Very short-type questions and answers
- 2A. Fill in the blanks
- 2B. True or false
- 2C. Single word questions and answers
3. Short-type questions and answers
4. Long-type questions and answers

1. Multiple choice questions and answers (MCQs)

1. Holandric genes are found where?
(a) X chromosome
(b) Y chromosome
(c) Both X and Y
(d) Autosomes

Ans: (b) Y chromosome

2. What are genetic disorders?
(a) Harmless
(b) Inherited
(c) Contagious
(d) Lethal

Ans: (b) Inherited

3. Which type of trait is colour blindness?
(a) Sex-linked dominant
(b) Autosomal dominant
(c) Sex-linked recessive
(d) Autosomal recessive

Ans: (c) Sex-linked recessive

4. What type of disease is thalassemia?
(a) Infectious
(b) Curable
(c) Environmental
(d) Genetic

Ans: (d) Genetic

5. Which chromosome carries sex-linked genes?
(a) Autosomes
(b) Y chromosome
(c) X chromosome
(d) None of these

Ans: (c) X chromosome

6. What are other names for red–green colour blindness?
(a) Protanopia
(b) Deuteranopia
(c) Phobia
(d) Both A & B

Ans: (d) Both A & B

7. From whom does a colourblind man get the gene?
(a) Grandmother
(b) Grandfather
(c) Mother
(d) Father

Ans: (c) Mother

8. Daughters of a haemophilic father and a normal mother are —
(a) Normal
(b) Carrier
(c) Haemophilic
(d) All of these

Ans: (b) Carrier

9. What happens in haemophilia?
(a) Blood does not clot after injury
(b) Blood clots very slowly
(c) Blood clots inside vessels
(d) Blood cell count becomes low

Ans: (b) Blood clots very slowly

10. What is another name for Cooley’s anaemia?
(a) Alpha thalassemia major
(b) Beta thalassemia major
(c) Beta thalassemia minor
(d) Hydrops fetalis

Ans: (b) Beta thalassemia major

11. Hemophilia usually follows —
(a) Autosomal dominant inheritance
(b) Autosomal recessive inheritance
(c) X-linked dominant inheritance
(d) X-linked recessive inheritance

Ans: (d) X-linked recessive

12. Which protein is defective in sickle cell anaemia?
(a) Hemoglobin
(b) Insulin
(c) Melanin
(d) Collagen

Ans: (a) Hemoglobin

13. Turner syndrome is caused by —
(a) XXY
(b) Trisomy 18
(c) Monosomy X
(d) XXX

Ans: (c) Monosomy X

14. Which disorder stops blood from clotting properly?
(a) Hemophilia
(b) Thalassemia
(c) Colour blindness
(d) Cystic fibrosis

Ans: (a) Hemophilia

15. A disorder caused by mutation in a single gene is —
(a) Diabetes
(b) Huntington’s disease
(c) Asthma
(d) Cancer

Ans: (b) Huntington’s disease

16. Which genotype causes Klinefelter syndrome in males?
(a) XY
(b) XX
(c) XO
(d) XXY

Ans: (d) XXY

17. Which disorder is inherited through mitochondrial DNA?
(a) Hemophilia
(b) Leber’s hereditary optic neuropathy (LHON)
(c) Huntington’s disease
(d) Tay–Sachs disease

Ans: (b) Leber’s hereditary optic neuropathy

18. Thalassemia mainly affects the production of —
(a) Insulin
(b) Platelets
(c) White blood cells
(d) Hemoglobin

Ans: (d) Hemoglobin

19. Most genetic disorders are caused by —
(a) Mutation
(b) Chromosomal problems
(c) Diet
(d) Both A & B

Ans: (d) Both A & B

20. A patient has a disease, and doctor says they can have only boys. This disease is —
(a) Y-linked
(b) Autosomal dominant
(c) X-linked dominant
(d) X-linked recessive

Ans: (c) X-linked dominant

21. Which of the following is a polygenic disorder?
(a) Red–green colour blindness
(b) Albinism
(c) Essential hypertension
(d) Trisomy 21

Ans: (c) Essential hypertension

22. Which disorder happens because a gene has too many repeats?
(a) Duchenne muscular dystrophy
(b) Huntington’s disease
(c) Phenylketonuria
(d) Tay-Sachs disease

Ans: (b) Huntington’s disease

23. What type of genetic disorder is Duchenne muscular dystrophy?
(a) Autosomal recessive
(b) Autosomal dominant
(c) X-linked recessive
(d) Mitochondrial inheritance

Ans: (c) X-linked recessive

24. What is a common feature of people with Marfan syndrome?
(a) Short height + learning problems
(b) Tall height + long arms and legs
(c) Skin rash + joint pain
(d) Facial deformities

Ans: (b) Tall height + long limbs

25. Which disorder has only one X chromosome?
(a) Turner syndrome
(b) Down syndrome
(c) Klinefelter syndrome
(d) Cystic fibrosis

Ans: (a) Turner syndrome

26. Which disorder means having extra sets of chromosomes?
(a) Down syndrome
(b) Klinefelter syndrome
(c) Turner syndrome
(d) Polyploidy

Ans: (d) Polyploidy

27. Which disorder harms the lungs and digestion?
(a) Cystic fibrosis
(b) Sickle cell anemia
(c) Down syndrome
(d) Hemophilia

Ans: (a) Cystic fibrosis

28. What is true about Hemophilia?
(a) It is autosomal dominant
(b) Caused by lack of clotting factors
(c) Affects men and women equally
(d) It is a lung disorder

Ans: (b) Caused by lack of clotting factors

29. Which disorder shows loss of motor control in adulthood?
(a) Huntington’s disease
(b) Down syndrome
(c) Tay-Sachs disease
(d) Duchenne muscular dystrophy

Ans: (a) Huntington’s disease

30. What is a correct treatment for sickle cell anemia?
(a) Antibiotics
(b) Bone marrow transplant
(c) Enzyme replacement therapy
(d) Surgery

Ans: (b) Bone marrow transplant

31. What is it called when parents’ traits mix together?
(a) Inheritance
(b) Particulate inheritance
(c) Blending inheritance
(d) Variation

Ans: (c) Blending inheritance

32. Variations are mainly caused by —
(a) Evolution
(b) Adaptation
(c) Asexual reproduction
(d) Mutation

Ans: (d) Mutation

33. An organism with two same alleles is —
(a) Heterozygous
(b) Heterologous
(c) Homozygous
(d) Homologous

Ans: (c) Homozygous

34. A diploid chromosome number is —
(a) N
(b) 2N
(c) 3N
(d) 2N + 1

Ans: (d) 2N + 1

35. Where is a gene’s position on a chromosome?
(a) Locus
(b) Allele
(c) Trait
(d) Variety

Ans: (a) Locus

36. What name do we use today for Mendel’s “factors”?
(a) Alleles
(b) Genes
(c) Mutants
(d) Loci

Ans: (b) Genes

37. Which trait shows up in a hybrid?
(a) Allele
(b) Hybrid
(c) Dominant
(d) Recessive

Ans: (c) Dominant

38. What do we call the outside look of a trait?
(a) Phenotype
(b) Genotype
(c) Trait
(d) Allele

Ans: (a) Phenotype

39. Cross between RR (red) × rr (white) gives —
(a) Half red
(b) Half white
(c) All red
(d) All pink

Ans: (c) All red

40. Which cross proves independent assortment?
(a) Back cross
(b) Test cross
(c) Monohybrid cross
(d) Dihybrid cross

Ans: (b) Test cross

41. Which percent of the F₂ plants show the recessive trait in a monohybrid cross?
(a) 50%
(b) 25%
(c) 100%
(d) 75%

Ans: (d) 75%

2. Very short-type questions and answers

2A. Fill in the blanks

1. A zygote having two identical alleles is called _____________. (Homozygous)
2. A _____________ is formed by the union of two gametes carrying contrasting alleles. (Hybrid)
3. The parental character, which is not expressed in F1 generation of monohybrid cross, is called ________________ character.(Recessive)
4. If the synthesis of ____________ is hampered a person develops severe anaemia. (Hemoglobin)
5. Regular blood transfusion increases _______________ content in the body of a thalassemia patient. (Iron)
6. Haemophilia is a ____________ chromosome linked genetic disease. (X)
7. Almost __________ % haemophilia ia caused by the deficiency of AHF. (80%)
8. The colour detecting cells of retina are known as _____________ cells. (Cone)
9. The gene responsible for blue colour blindness is a/an ___________ gene. (X-linked recessive)
10. Only ___________ can be carriers in care of genetic disease. (Females)
11. The specific location of a gene on a chromosome is called _____________ of the gene.(Locus)
12. Free lobed ear is formed by a pair of ___________ genes. (Dominant)
13. Mutation generates _______________. (Variation)
14. Individual containing Bb genotype is ______________ type of individual. (Heterozygous)
15. In heterozygous organism _______________ character is expressed.(Dominant)
16. Mendel’s first law of heredity is known as the law of ________________.(Segregation)
17. The parental generation of monohybrid cross is marked as __________ generation. (P)
18. To get F2 generation, Mendel allowed pollination ____________ among F1 plants. (Self-pollination)
19. Thalassemia is a _______________ disease. (Genetic)
20. In _______________ disease O2 transport capacity decreases. (Sickle cell anaemia)

2B. True or false

1. Mendel is the father of genetics.(True)
2. The external expression of a character is called phenotype.(True)
3. T and t allele together from a homozygote.(False)
4. Sudden heritable permanent change of chromosome or gene is called variation.(False)
5. Scientific name of evening primorse is Mirabilis Jalapa. (False)
6. In evening primorse plant incomplete dominane is seen. (True)
7. Malaria AIDS hepatitis etc are examples of genetic diseases.(False)
8. In thalassemia bone marrow proliferates at an excessive rate.(True)
9. In thalassemia, synthesis of globin protein is hampered.(True)
10. Haemophilia is an autosome linked genetic disorder. (False)
11. Haemophilia A is caused by restricted synthesis of factor X. (False)
12. Night blindness is an X chromosome linked disease. (False)
13. Rod cells of human retina help in colour detection.(False)
14. 5-8% male of the total global populization are colour blind. (True)
15. In F2 generation of Mendel’s dihybrid cross, both phenotypic and genotypic ratio is 9:3:3:1. (False)
16. In Mendel’s monohybrid cross, all F1 offsprings have pure dominant character. (False)
17. In genetics, cross denotes the union of a male and female gamete. (True)
18. Round seed is a recessive character. (False)
19. In pea plant both selfing and cross pollination can be done. (True)
20. Mendel obtained 75% tall plant in F1 generation of monohybrid cross. (False)

2C. Single word questions and answers

1. What does Tt indicate in Mendel’s experiment?
Ans: Tt indicates a heterozygous tall plant.

2. What does TT denote in Mendel’s experiment?
Ans: TT denotes a homozygous tall plant.

3. What does tt denote in Mendel’s experiment?
Ans: tt denotes a homozygous dwarf plant.

4. Mention one character and its opposite trait in guinea pig.
Ans: Black coat (dominant) and white coat (recessive).

5. Who is called the father of genetics?
Ans: Gregor Johann Mendel is called the father of genetics.

6. Mendel’s excellent experiments, analysis and laws are together called what?
Ans: Mendel’s experiments, analysis and laws are called Mendelism.

7. What is the process of passing traits from one generation to the next?
Ans: The process of transferring traits from parents to offspring is called inheritance.

8. Who coined the term “Genetics”?
Ans: William Bateson coined the term genetics.

9. On which organism did Mendel perform his experiments?
Ans: Mendel performed his experiments on the garden pea plant.

10. What is the result of Mendel’s monohybrid cross?
Ans: The result is a 3:1 phenotypic ratio and 1:2:1 genotypic ratio in the F₂ generation.

11. What is the result of Mendel’s dihybrid cross?
Ans: The result is a 9:3:3:1 phenotypic ratio in the F₂ generation.

12. How many pairs of contrasting characters did Mendel study?
Ans: Mendel studied seven pairs of contrasting characters.

13. What does TT denote in Mendel’s experiment?
Ans: TT denotes a homozygous tall plant.

14. What does Tt denote in Mendel’s experiment?
Ans: Tt denotes a heterozygous tall plant.

15. What is the condition called when offspring differ from parents?
Ans: When the offspring show differences compared to their parents, it is called variation.

16. What is the character that is visible externally called?
Ans: The character of an organism that can be observed externally is called the phenotype.

17. Name two high-yield, disease-resistant paddy plants made by hybridization.
Ans: IR-8 and Ratna are two high-yield, disease-resistant paddy varieties.

18. Name two high-yield wheat plants produced by hybridization.
Ans: Sonora-64 and Kalyan Sona are high-yielding wheat varieties.

19. Name two artificially bred cows that give large quantities of milk.
Ans: Jersey and Holstein-Friesian are high milk-yielding breeds of cows.

20. How many chromosomes are present in a human being?
Ans: A human has 46 chromosomes.

21. What guinea pig appears in F₁ when pure white is crossed with pure black?
Ans: When a pure white guinea pig is crossed with a pure black one, all the F₁ guinea pigs are black.

22. In Mendel’s experiment what does Tt denote?
Ans: In Mendel’s experiment, Tt denotes a hybrid tall plant.

23. Name a paddy plant produced by hybridization.
Ans: IR-8 is a paddy plant produced by hybridization.

24. Which law did Mendel find from his monohybrid cross?
Ans: Mendel found the Law of Segregation.

25. Which law did Mendel derive from his dihybrid cross?
Ans: Mendel derived the Law of Independent Assortment.

26. Name one congenital disease.
Ans: Heart defect at birth is an example of a congenital disease.

27. Out of AA, Aa, aa, bb, Bb, BB which are heterozygous?
Ans: Aa and Bb are heterozygous genotypes.

28. What do you mean by genome?
Ans: A genome is the complete set of genes present in an organism.

29. What do you mean by parental generation?
Ans: The parental generation is the first generation of parents used in a cross.

30. Where are genes located?
Ans: Genes are located on chromosomes.

31. What causes phenotypic and genotypic differences among individuals?
Ans: Sexual reproduction and random mutations cause these differences.

32. What is Mendelism?
Ans: Mendelism refers to Mendel’s laws and principles of inheritance.

33. Which character is expressed in the F₁ generation of a monohybrid cross?
Ans: The dominant character is expressed in the F₁ generation.

34. Which character is not expressed in the F₁ generation?
Ans: The recessive character is not expressed in the F₁ generation.

35. Which ratio shows the genetic makeup (genotype) of offspring?
Ans: The genotypic ratio shows the genetic constitution.

36. What is locus?
Ans: A locus is the fixed position of a gene on a chromosome.

37. Give an example of an inherited variation in humans.
Ans: Eye colour variation is an inherited variation in humans.

38. Which sperm results in the birth of a girl child?
Ans: The X-carrying sperm leads to the birth of a girl child.

39. Which ratio shows the visible characters of offspring?
Ans: The phenotypic ratio.

40. In which journal was Mendel’s work published?
Ans: It was published in the Proceedings of the Natural History Society of Brünn.

41. What does the ratio 3:1 indicate?
Ans: It indicates three dominant traits and one recessive trait in F₂.

42. How much oxygen is carried by 1 gram of haemoglobin?
Ans: 1 gram of haemoglobin carries about 1.34 ml of oxygen.

43. What does the ratio 1:2:1 indicate?
Ans: It indicates one homozygous dominant, two heterozygous, and one homozygous recessive offspring

44. Who first designed the checkerboard?
Ans: Reginald C. Punnett first designed the checkerboard, known as the Punnett square.

45. What does Mendelian factors mean?
Ans: Mendelian factors mean genes that control hereditary characters.

46. What is the genotypic ratio in the F₂ generation of a monohybrid cross?
Ans: The genotypic ratio is 1:2:1.

47. Which law did Mendel conclude from his dihybrid cross?
Ans: Mendel concluded the Law of Independent Assortment.

48. Name the group of diseases caused by defective haemoglobin.
Ans: These diseases are called haemoglobinopathies.

49. What is α-thalassemia?
Ans: α-thalassemia is a disorder where the alpha-chain of haemoglobin is not formed properly.

50. Name the disease caused by deficiency of clotting factor IX.
Ans: Hemophilia B is caused by deficiency of factor IX.

3. Short-type questions and answers

1. What do you mean by the term homozygous?
Ans: Homozygous means having two identical alleles of a gene for a particular trait, like TT or tt. Such individuals produce only one type of gamete for that trait.

2. What is homozygous condition?
Ans: Homozygous condition occurs when an organism has two same alleles for a trait, like TT or tt. It expresses the same character in all conditions.

3. What do you mean by Mendelian error?
Ans: A Mendelian error is a deviation from the expected Mendelian pattern of inheritance, usually caused when a child’s genetic information does not match the predicted pattern from the parents.

4. What do you mean by the term locus?
Ans: Locus refers to the specific fixed position of a gene on a chromosome. Every gene has a particular locus.

5. What is recessive character?
Ans: A recessive character is a trait that remains hidden in the F₁ generation and only appears when both alleles are recessive (tt).

6. Write the practical applications of genetics in human welfare with one example from plant and one example from animal.
Ans: Genetics helps improve plant and animal varieties.

Plant example: IR-8 paddy is high-yielding.
Animal example: Jersey cow gives more milk due to genetic improvement.

7. Name few congenital diseases.
Ans: Congenital diseases include cleft lip, congenital heart defect, thalassemia, and Down syndrome.

8. Why are Watson, Crick, and Wilkins famous?
Ans: They are famous for discovering the double helix structure of DNA, which explained how genetic information is stored and passed on.

9. What do you mean by Genetic Engineering?
Ans: Genetic engineering is the artificial modification of genes using biotechnology. It allows scientists to change, add, or remove genes.

10. What do you mean by the term Genome?
Ans: A genome is the complete set of genes or genetic material present in an organism.

11. What do you mean by sexual and asexual chromosomes?
Ans:

Type of Chromosome	Meaning
Sexual chromosomes	These are the X and Y chromosomes that determine the sex of an individual.
Asexual chromosomes	These are the autosomes, which carry genes for traits other than sex determination.

12. Write a short note on Hybridization.
Ans: Hybridization is the crossing of two genetically different parents to produce offspring with desirable traits like high yield or disease resistance.

13. Phenotypic ratio of round to wrinkled seeds in F₂?
Ans: The ratio is 3 round : 1 wrinkled, showing dominance of round seeds.

14. What types of plants will be obtained in F₁ when pure white × pure red gives only red?
Ans: All F₁ plants will have red flowers, as red is a dominant trait.

15. What is a gene and where is it present?
Ans: A gene is a unit of heredity that controls a character. It is present on chromosomes.

16. Name the scientists who rediscovered Mendelism.
Ans: Mendelism was rediscovered by de Vries, Correns, and Tschermak.

17. What do you mean by recessive character? Give one animal example.
Ans: A recessive character appears only when both alleles are recessive. Example: white coat colour in guinea pig.

18. Differentiate between the terms homozygous and heterozygous.

Homozygous: Same alleles (TT or tt).
Heterozygous: Different alleles (Tt).

19. What are the differences between homozygote and heterozygote?

Homozygote produces one type of gamete.
Heterozygote produces two types of gametes.

20. When RRYY is crossed with rryy, what is the F₁ genotype? F₂ phenotypic ratio?
Ans:

F₁ genotype: RrYy
F₂ phenotypic ratio: 9:3:3:1

21. State two conclusions from Mendel’s monohybrid cross.
Ans:

Characters are controlled by pairs of factors (genes).
The recessive trait reappears in F₂ in 1:2:1 ratio.

22. What do you mean by incomplete dominance?
Ans: In incomplete dominance, one allele does not completely dominate the other, so the F₁ generation shows an intermediate form of the trait. For example, crossing red and white Mirabilis flowers produces pink flowers.

23. Why did Mendel select pea plants for his experiments?
Ans: Mendel selected pea plants because they have clear contrasting traits, a short life cycle, produce many seeds, and can undergo both self-pollination and cross-pollination. These qualities made them very suitable for inheritance studies.

4. Long-type questions and answers

1. (a) What is monohybrid cross? (b) Explain the law of Mendel with the help of monohybrid cross.
Ans:
(a) A monohybrid cross is a genetic cross between two parents that differ in one pair of contrasting characters, such as tall and dwarf plants.
(b) Mendel crossed a pure tall pea plant (TT) with a pure dwarf plant (tt). All F₁ plants were tall (Tt). This proved the Law of Dominance. When F₁ plants were self-crossed, the F₂ generation showed three tall and one dwarf (3:1 ratio). Their genotypes were 1 TT : 2 Tt : 1 tt, proving the Law of Segregation. The reappearance of the recessive dwarf character in F₂ shows that the traits do not mix and remain separate in gametes.

2. Explain the terms phenotype and genotype with the help of Mendel’s monohybrid cross experiment.
Ans: Phenotype refers to the external and observable characters of an organism, such as tall or dwarf.
Genotype refers to the genetic constitution of an organism, such as TT, Tt, or tt, which determines the phenotype. In Mendel’s monohybrid cross, the F₁ plants were all tall, but their genotype was Tt. In the F₂ generation, the ratio of phenotypes (tall : dwarf) was 3 : 1, but the genotypic ratio was 1 TT : 2 Tt : 1 tt.
Thus, tall plants in the F₂ generation had two different genotypes (TT and Tt) even though their phenotype was the same. This clearly shows the difference between phenotype (appearance) and genotype (genetic makeup).

3. Explain the ratio of pure red and pure white flowers in F₂ when pure red is crossed with pure white.
Ans: In pea plants, red flower colour (R) is dominant over white (r). When a pure red plant (RR) is crossed with a pure white plant (rr), all the F₁ plants are red (Rr).
When the F₁ plants are self-crossed (Rr × Rr), the F₂ generation produces three types of genotypes: 1 RR, 2 Rr, and 1 rr. Here:

RR = pure red
Rr = hybrid red
rr = pure white

Therefore, the ratio of pure red to pure white in the F₂ generation is 1 : 1, although the overall phenotypic ratio is 3 red : 1 white. This result supports dominance, segregation, and reappearance of the recessive trait in the F₂ generation.

4. What will be your comment derived from the monohybrid cross?
Ans: The monohybrid cross shows that each character is controlled by a pair of alleles. In the F₁ generation, the dominant allele expresses itself, while the recessive allele remains hidden. In the F₂ generation, the recessive trait reappears in 1 out of 4 individuals. The results give a 3 : 1 phenotypic ratio and a 1 : 2 : 1 genotypic ratio. This experiment clearly proves that alleles do not blend, but separate or segregate during gamete formation, which supports Mendel’s Law of Segregation.

5. (a) Write the scientific name of Drosophila. (b) Explain monohybrid cross of Drosophila through a checkerboard.
Ans:
(a) The scientific name of Drosophila is Drosophila melanogaster, commonly called the fruit fly.
(b) In Drosophila, red eye colour (R) is dominant over white eye colour (r). When a pure red-eyed fly (RR) is crossed with a pure white-eyed fly (rr), all the F₁ flies have red eyes (Rr). Using a checkerboard (Punnett square) for the F₂ cross (Rr × Rr), the genotypes obtained are:
RR, Rr, rR, and rr. Thus, three flies have red eyes and one fly has white eyes, giving a 3 : 1 phenotypic ratio.
This result supports Mendel’s Law of Segregation.

6. Discuss the application of the science of heredity in human welfare.
Ans: The science of heredity is useful in improving plants, animals and human health. In agriculture, high-yield and disease-resistant varieties like IR-8 rice are produced. In animal husbandry, improved breeds such as Jersey cows give more milk. In medicine, heredity helps in understanding genetic diseases like haemophilia and Down syndrome. Biotechnology uses genetic principles to produce insulin, vaccines and useful hormones. Thus, heredity plays an important role in increasing food production, livestock improvement and medical progress.

7. Why the following scientists are famous? (a) Muller (b) Watson, Crick and Wilkins (c) Hargobind Khorana
Ans: H. J. Muller is famous because he discovered that X-rays can induce mutations in genes, proving that external agents can alter hereditary material. Watson, Crick and Wilkins became world-renowned for discovering the double helix structure of DNA, which explained how genetic information is stored and passed from one generation to the next. Dr. Hargobind Khorana is known for decoding the genetic code and showing how sequences of bases in DNA direct the formation of proteins. His pioneering work in molecular genetics earned him the Nobel Prize.

8. Explain the dihybrid cross of Mendel with an example from Guinea pig.
Ans: In guinea pigs, the black coat colour (B) is dominant over white (b), and short hair (S) is dominant over long hair (s). When a pure black-short animal (BBSS) is crossed with a pure white-long animal (bbss), all the F₁ offspring produced are black-short (BbSs), showing both dominant traits. When these F₁ hybrids are self-crossed, the F₂ generation displays four types of offspring in the classical dihybrid ratio of 9:3:3:1. These groups include nine black-short, three black-long, three white-short and one white-long guinea pig. This result demonstrates Mendel’s Law of Independent Assortment, which states that the inheritance of one character (coat colour) occurs independently of the inheritance of another character (hair length).

9. (a) Law of Independent Assortment. (b) Ratio in F₂ for pure red × pure white.
Ans: According to the Law of Independent Assortment, when two or more pairs of characters are studied together, the factors (alleles) of each pair separate independently during gamete formation. This leads to new combinations of traits in the offspring. When pure red-flowered plants (RR) are crossed with pure white-flowered plants (rr), all F₁ plants are red (Rr). In the F₂ generation, the segregation of alleles produces a genotypic ratio of 1 RR : 2 Rr : 1 rr and a phenotypic ratio of 3 red : 1 white. This experiment clearly shows that dominant and recessive alleles segregate independently during gamete formation.

10. State four pairs of contrasting characters and inference from monohybrid cross.
Ans: Mendel selected several pairs of contrasting characters in pea plants. Four examples are tall versus dwarf plants, round versus wrinkled seeds, yellow versus green seeds and axial versus terminal flowers. From the results of the monohybrid cross, Mendel inferred that each character is controlled by a pair of factors or alleles. These factors do not blend but segregate during gamete formation. The dominant character appears in all F₁ plants, while the recessive character reappears in the F₂ generation in a phenotypic ratio of 3:1. These observations led to the formulation of the Law of Segregation.

11. (a) What is dihybrid cross? (b) Explain the dihybrid cross with a checkerboard.
Ans: A dihybrid cross is one in which two contrasting characters are studied simultaneously to understand their pattern of inheritance. When pure dominant parents (RRYY) are crossed with pure recessive parents (rryy), all the F₁ plants produced are RrYy. When these F₁ plants are selfed, the F₂ generation contains sixteen possible combinations, which may be shown with the help of a checkerboard. These combinations produce a phenotypic ratio of 9:3:3:1. This classical ratio confirms Mendel’s Law of Independent Assortment, showing that the inheritance of one character occurs independently of another.